In recent years, the development of modern technology has forever changed the way people live. In particular, the overwhelming use of wireless communication devices has made the communication between people a much easier task. In addition, a variety of functions, such as game, phone book, voice mail system, calculator, alarm, wireless application protocol (WAP), text messaging, electronic mail, etc., has also been included in the wireless communication device to provide users with additional services. Following this trend, a multi-function wireless communication device is developed to satisfy the users' need.
Since more and more features are integrated into the wireless communication device, the requirement of battery capacity has also been increased. The capacity of the conventional lithium battery no longer sustains the normal usage of the wireless communication device, especially for the wireless fidelity (Wi-Fi) communication devices. Since the Wi-Fi communication device occurs a lot of power consumption, it is deemed necessary to develop low power consumption Wi-Fi communication devices.
Referring to FIG. 1, a signal emission circuit of a commercially available Wi-Fi communication device is illustrated. The signal emission circuit comprises a radio frequency (RF) auto-gain control circuit. The circuit includes a dynamic gain power amplifier 11, a power detector, a comparator 13, a low pass filter 14, wherein the output voltage VDET of detector 12 is proportional to the output power POUT (in the unit of dBm (decibel per milliwatt)), F(S) is a function of the filter 14, Vc is the control voltage, and VREF is the reference voltage, the value of which is 15 dBm. In this manner, the detector 12 detects the output power POUT of the amplifier 11, and outputs a voltage VDET after the detection of the detector 12. The output voltage VDET further is inputted into the comparator 13 and is compared with the reference voltage VREF (15 dBm). Later, the filter 14 filters out the noise of the control voltage Vc output from the comparator 13, thereby making the control voltage Vc be the control value of the gain of the amplifier 11.
With respect to the conventional Wi-Fi communication device, the output power is usually set to be 15 dBm. Therefore, the reference voltage VREF (15 dBm) is a reference voltage corresponding to 15 dBm. The basic principle of the auto-gain control circuit can thus be described by using the following formulae:                1) Output voltage VDET=A×POUT (dBm), where A is the gain of the detector 12;        2) Control voltage Vc=F(S) (VREF (15 dBm)−VDET).        
Consequently, if the output power is smaller than 15 dBm, VDET is smaller than VREF, which makes Vc positive, thereby increasing the power amplification gain. The POUT then approaches to the output power originally configured. On the contrary, if the output power reaches 15 dBm, Vc will become negative, which will decrease the power amplification gain. Finally, POUT=15 dBm.
According to the description set forth above, the conventional Wi-Fi communication device emits the radio frequency signals with the same power, regardless of the signal strength of where the Wi-Fi communication device is located. This gives rise to a poor talk quality when the signals are weaker (or even no signal is received). When the signal strength is stronger, a lot of battery power is wasted, which gives rise to a shorter usage time. For this reason, it is deemed necessary to develop a Wi-Fi communication device that consumes less power, so as to overcome the conventional drawbacks.